Functions of the circulation

To serve the needs of the tissues:

1- Transport nutrients & remove waste products.

2- Transport hormones, enzymes, body heat, electrolytes ---etc.

3- Maintain normal homeostasis for optimal survival &function of cells.

Functional Parts of the circulation

1- Arteries:

Has strong muscular walls. Transport blood

Rapidly under high pressure to the tissues.

2- Arterioles:

Has strong muscular walls that can close the

arteriole completely or dilate it several folds i.e.

they alter blood flow to the capillaries in

Response to needs.

Arterioles & small arteries are called

(Resistance vessels).

3- Capillaries: Very thin wall (unicellular layer of endothelial cells).Very small internal diameter. Have numerousCapillary pores and very large surface area(exchange blood vessels).For exchange of gases, nutrients, waste productsetc.

Blood flow is intermittent, turn on and off every few seconds or minutes (vasomotion), determined by oxygen demand.

Capillary Structure. (a) A continuous capillary. The enlargement shows Capillary Structure. (a) A continuous capillary. The enlargement shows routes for the diffusion of water and solutes. (b) A fenestrated capillary. routes for the diffusion of water and solutes. (b) A fenestrated capillary. Note the pores, which facilitate diffusion across the endothelial lining.Note the pores, which facilitate diffusion across the endothelial lining.

The Organization of a Capillary Bed. (a) A typical capillary bed. Solid The Organization of a Capillary Bed. (a) A typical capillary bed. Solid arrows indicate consistent blood flow; dashed arrows indicate variable arrows indicate consistent blood flow; dashed arrows indicate variable or pulsating blood flow. (b) A micrograph of a number of capillary beds.or pulsating blood flow. (b) A micrograph of a number of capillary beds.

4- Venules & Veins:

(Capacitance Vessels)

At rest more than ⅔rds. of total blood

volume is found within the venous system.

More than half of it is within venules.

Relationships among Vessel Relationships among Vessel Diameter, Cross-Sectional Diameter, Cross-Sectional Area, Blood Pressure, and Area, Blood Pressure, and Blood Velocity.Blood Velocity.

Capillary pressures and capillary exchange

Capillary exchange plays important role in

homeostasis.

The important processes that move materials

across capillary walls are:

1- Diffusion

2- Filtration

3- Reabsorption

Capillary Filtration. Capillary hydrostatic pressure (CHP) forces water and Capillary Filtration. Capillary hydrostatic pressure (CHP) forces water and solutes through the gaps between adjacent endothelial cells in continuous solutes through the gaps between adjacent endothelial cells in continuous capillaries. The sizes of solutes that move across the capillary wall are capillaries. The sizes of solutes that move across the capillary wall are determined primarily by the dimensions of the gaps.determined primarily by the dimensions of the gaps.

Forces Acting across Capillary Walls. At the arterial end of the capillary, Forces Acting across Capillary Walls. At the arterial end of the capillary, capillary hydrostatic pressure (CHP) is greater than blood colloid osmotic capillary hydrostatic pressure (CHP) is greater than blood colloid osmotic pressure (BCOP), so fluid moves out of the capillary (filtration). Near the pressure (BCOP), so fluid moves out of the capillary (filtration). Near the venule, CHP is lower than BCOP, so fluid moves into the capillary venule, CHP is lower than BCOP, so fluid moves into the capillary (reabsorption). In this model, interstitial fluid colloid osmotic pressure (ICOP) (reabsorption). In this model, interstitial fluid colloid osmotic pressure (ICOP) and interstitial fluid hydrostatic pressure (IHP) are assumed to be 0 mmHg and and interstitial fluid hydrostatic pressure (IHP) are assumed to be 0 mmHg and so are not shown.so are not shown.

More filtration than reabsorption occursalong the capillary. The extra amount of fluid in the interstitialspaces is carried by the lymphatic Vessels→ venous circulation. This will help in:1- Constant exchange of fluid.2- Accelerate distribution of substances.3- Transport insoluble lipids & tissue proteins.4- Carry bacterial toxins to lymphoid tissues →

provide immunity.

Edema

Is an excessive amount of fluid in the interstitialSpaces caused by:

1- ↑ Capillary hydrostatic pressure e.g. heart failure, local venous block, failure of venous pumps, etc.

2- ↓ Plasma proteins due to e.g: A- Nephrosis. B- Burns & wounds. C- Liver disease D- Malnutrition.

3- ↑ Capillary permeability e.g. in allergic reactions

& burns.

4- Blockage of lymph return by, e.g. infection or cancer.

Causes of edema

1- Increased hydrostatic blood pressureheart failure (left or right),

excess fluid in the blood.

2- Decreased blood osmotic pressureLiver, kidney diseases,

malnutrition (kwashiorkor), burn injuries.

3- Increased interstitial hydrostatic pressure (lymphatic capillary blockage)breast cancer surgery,

Elephantiasis.

4- Leaking capillary wallhistamine release during

allergic reaction.